A capacitive touch sensor, referred to simply as a touch sensor in the following, may detect the presence and location of a touch or the proximity of an object (such as a user's finger or a stylus) on a surface. Touch sensors are often combined with a display to produce a touch screen. In other devices, the touch sensors are not combined with a display, e.g. a touch pad of a laptop computer. A touch screen enables a user to interact directly with what is displayed on the screen through a graphical user interface (GUI), rather than indirectly with a mouse or touch pad. A touch sensor may be attached to or provided as part of a mobile phone, tablet or laptop computer, for example.
Touch sensors may be classified into grid and matrix types. In a matrix type, an array of electrodes is arranged on the surface which are electrically isolated from each other, so that each electrode in the array provides its own touch signal. A matrix type touch sensor is therefore naturally suited to situations in which an array of touch-sensitive buttons are needed, such as in a control interface, data entry interface or calculator. In a grid type, there are two groups of parallel electrodes, usually referred to as X and Y electrodes, since they are typically arranged orthogonal to each other. A number of nodes are defined by the crossing points of pairs of X and Y electrodes (as viewed in plan view), where the number of nodes is the product of the number of X electrodes and Y electrodes. A grid type touch sensor is the type typically used for touch screens on mobile phones, drawing tablets and so forth. In earlier designs, the X and Y electrodes are arranged either side of a dielectric layer, so they are vertically offset from each other by the thickness of the dielectric layer, vertical meaning orthogonal to the plane of the stack layers. In more recent designs, to reduce stack thickness, the X and Y electrodes are deposited on the same side of a dielectric layer, i.e. in a single layer, with thin films of dielectric material being locally deposited at the cross-overs to avoid shorting between the X and Y electrodes. A single electrode layer design of this kind is disclosed in US 2010/156810 A1, the entire contents of which are incorporated herein by reference.
Touch sensors may also be classified into self capacitance and mutual capacitance types.
In a self capacitance measurement, the capacitance being measured is between an electrode under a dielectric touch panel and the touching finger, stylus etc., or more precisely the effect that the touch's increase in capacitance with the electrode has on charging a measurement capacitor that forms part of the touch IC's measurement circuit. The finger and the electrode can thus be thought of as acting as the plates of a capacitor with the touch panel being the dielectric.
In a mutual capacitance measurement, adjacent pairs of electrodes are arranged under the touch panel, and form the nominal plates of the capacitor. A touching body acts to modify the capacitance associated with the electrode pair by replacing what was the ambient environment, i.e. in most cases air, but possibly water or some other gas or liquid, with the touching object, which may be effectively a dielectric material (e.g. a dry finger, or a plastics stylus) or in some cases could be conductive (e.g. a wet finger, or a metal stylus). One of the pair of electrodes is driven with a drive signal, e.g. with a burst of pulses, and the other electrode of the pair senses the drive signal. The effect of the touch is to attenuate or amplify the drive signal received at the sense electrode, i.e. affects the amount of charge collected at the sense electrode. Changes in the mutual capacitance between a drive electrode and a sense electrode provide the measurement signal. It is noted that in a mutual capacitance grid sensor, there is a convention to label drive electrodes as the X electrodes and sense electrodes as the Y electrodes, although this choice is arbitrary. A perhaps clearer labelling that is often used is to label the drive electrodes as “Tx” for transmission and the sense electrodes as “Rx” for receiver in analogy to telecoms notation, although this labelling is of course specific to mutual capacitance measurements.
Current industry standard touch screens for mobile phones rely on operating the same touch sensor to make both self capacitance and mutual capacitance measurements, since acquiring both is beneficial to gaining additional information about the touch which can be used in post-processing to increase the reliability of interpretation. For example, mutual capacitance measurement have high noise immunity, whereas self capacitance measurements are easier to interpret and give a direct measure of moisture presence.
Currently, the most common display technologies that are integrated with touch sensors to form a touch screen are thin film transistor (TFT) liquid crystal displays (LCDs) and organic light emitting diode (OLED) displays, and the touch sensor design is a grid design operated to make both self capacitance and mutual capacitance measurements. The grid design patterns the X and Y electrodes in some way designed to achieve the best compromise of competing requirements, such as position sensitivity, lateral field uniformity (for mutual capacitance measurements), fast charging time and so forth. In particular, where the X and Y lines cross, the mutual capacitance is at its largest. To keep this capacitance as low as possible, it is therefore normal practice to narrow the X and Y lines where they cross to keep the area of the capacitor formed by the crossing as small as possible. However, there is a trade off, since these pinch points form the largest resistance elements, and thereby can become the rate limiting factor for charge times. Away from the XY crossing points, it is beneficial for the electrodes to spread out to more or less cover the whole panel sub-area associated with the node. These spread-out areas of the electrodes may be referred to as electrode pads. Having larger area pads improves signal strength for self capacitance measurements and, for mutual capacitance measurements, means that signal contributions can be obtained from touches across the node sub-area. A conventional electrode pattern therefore combines narrow crossing points and spread-out electrode pads in between crossing points.
FIG. 30 of the accompanying drawings shows in plan view a currently popular electrode pattern design for a hybrid self/mutual capacitance sensor, which is referred to as the diamond pattern in the art. US 2010/156810 A1 discloses diamond pattern touch sensors of this kind.
The electrode pattern comprises rows of parallel X lines X2, X3, X4, X5 (hatched) and orthogonal thereto columns of parallel Y lines (cross-hatched), Y3, Y4, Y5, Y6, Y7. The X and Y lines cross at nodes 28 where the X and Y lines are narrowed to respective widths WSX and WSY with the X line lying above the Y line. There is a vertical separation between the X and Y electrodes at the crossing point provided by a dielectric layer or film. The area of the crossing point is thus WSX·WSY. Each X electrode may be viewed as having a spine 30, and each Y electrode a spine 32. Away from the bridges, the electrodes expand out into square pads, which are referred to as diamonds since the square shape is arranged at 45 degrees to X and Y. Each electrode is thus a series of diamond-shaped pads interconnected with short bridging strips. A given node has an associated sub-area of the panel, which is illustrated for node (X3, Y5) with the box 27.
In a diamond pattern, the touch locations which are used as benchmarks in performance testing are as follows:                On Node: touching on a spine crossing-point 28;        On X: touching on the middle of a diamond in line with an X electrode spine 30, i.e. on the X electrode spine at the farthest point away from two adjacent crossing points, labelled 31 in the figure;        On Y: touching in the middle of a diamond in line with a Y electrode spine 32, i.e. on the Y electrode spine at the farthest point away from two adjacent crossing point, labelled 33 in the figure;        Off Node: touching at the farthest point away from the two adjacent X spines and two adjacent Y spines, labelled as 29 in the figure.        
With the diamond pattern, an “On X” or “On Y” touch represent the areas of lowest field strength for a mutual capacitance measurement, i.e. the lowest sensitivity. For other patterns, this statement can be generalised to the field strength, and hence sensitivity, being the lowest in the interior of electrodes the further the touch lies away from the gaps between paired drive and sense electrodes.
FIG. 31A is a schematic cross-section through a touch panel in a plane perpendicular to the plane of the stack showing a mutual capacitance measurement involving an individual pair of X (drive) and Y (sense) electrodes: Xn, Yn. Electric field lines are shown schematically with the arrow-headed, curved lines. As can be seen from the schematic depiction, field strength at the touch surface is highest in the region adjacent the gap between the X and Y electrodes and decreases towards the interior of each electrode.
FIG. 31B is a schematic cross-section through the same touch panel as FIG. 31A in the same plane showing a self capacitance measurement involving the same pair of X and Y electrodes: Xn, Yn. Electric field lines are shown schematically with the arrow-headed lines. As can be seen from the schematic depiction, field strength across the node area is substantially constant. In other words there is no, or only insubstantial, lateral field non-uniformity. It is also noted in passing, that in self capacitance mode, a small area touch On X (or On Y) totally confined within one of the X electrode pads (or Y electrode pads), e.g. at point 31 (or 33), will provide no signal for any of the Y electrodes (or X electrodes), so position information in the Y direction (or the X direction) will be completely absent from the signal.